6889300: assert(i != k || is_new || i->outcnt() > 0, "don't return dead nodes")

Summary: PhiNode::Ideal() should return TOP for Phi node with no users.
Reviewed-by: never, jrose

src/share/vm/opto/cfgnode.cpp

@@ -1531,6 +1531,8 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
     return NULL;                // No change
 
   Node *top = phase->C->top();
+  bool new_phi = (outcnt() == 0); // transforming new Phi
+  assert(!can_reshape || !new_phi, "for igvn new phi should be hooked");
 
   // The are 2 situations when only one valid phi's input is left
   // (in addition to Region input).
@@ -1550,6 +1552,12 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
     }
   }
 
+  if (can_reshape && outcnt() == 0) {
+    // set_req() above may kill outputs if Phi is referenced
+    // only by itself on the dead (top) control path.
+    return top;
+  }
+
   Node* uin = unique_input(phase);
   if (uin == top) {             // Simplest case: no alive inputs.
     if (can_reshape)            // IGVN transformation
@@ -1684,8 +1692,7 @@ Node *PhiNode::Ideal(PhaseGVN *phase, bool can_reshape) {
             // Equivalent code is in MemNode::Ideal_common
             Node *m  = phase->transform(n);
             if (outcnt() == 0) {  // Above transform() may kill us!
-              progress = phase->C->top();
-              break;
+              return top;
             }
             // If transformed to a MergeMem, get the desired slice
             // Otherwise the returned node represents memory for every slice

src/share/vm/opto/ifnode.cpp

@@ -240,13 +240,13 @@ static Node* split_if(IfNode *iff, PhaseIterGVN *igvn) {
   // as a single huge transform.
   igvn->register_new_node_with_optimizer( region_c );
   igvn->register_new_node_with_optimizer( region_x );
-  phi_x = phase->transform( phi_x );
   // Prevent the untimely death of phi_x.  Currently he has no uses.  He is
   // about to get one.  If this only use goes away, then phi_x will look dead.
   // However, he will be picking up some more uses down below.
   Node *hook = new (igvn->C, 4) Node(4);
   hook->init_req(0, phi_x);
   hook->init_req(1, phi_c);
+  phi_x = phase->transform( phi_x );
 
   // Make the compare
   Node *cmp_c = phase->makecon(t);
@@ -322,6 +322,7 @@ static Node* split_if(IfNode *iff, PhaseIterGVN *igvn) {
         phi_s = PhiNode::make_blank(region_s,phi);
         phi_s->init_req( 1, phi_c );
         phi_s->init_req( 2, phi_x );
+        hook->add_req(phi_s);
         phi_s = phase->transform(phi_s);
       }
       proj_path_data = phi_s;
@@ -333,6 +334,7 @@ static Node* split_if(IfNode *iff, PhaseIterGVN *igvn) {
         phi_f = PhiNode::make_blank(region_f,phi);
         phi_f->init_req( 1, phi_c );
         phi_f->init_req( 2, phi_x );
+        hook->add_req(phi_f);
         phi_f = phase->transform(phi_f);
       }
       proj_path_data = phi_f;